A glass plate to be used for a window of an automobile, is provided with a conductive printed wire as a heater wire for defogging or as an antenna wire for receiving radio, television or the like. Such conductive printed wire is provided mainly on a rear window or on a rear side window of an automobile. The conductive printed wire consists mainly of a fired product of a paste containing silver. Specifically, a paste having silver, glass frit, a binder resin and so on is printed on a glass plate surface in a predetermined pattern by screen printing and then the glass plate is heated to decompose the resin content of the binder and to fix silver on the glass plate by the glass frit, followed by firing silver to form a conductive printed wire on the glass plate surface.
There is a restriction to the voltage in the electrical system to be used for an automobile, and in order to obtain a desired heat generation, it is necessary to set the resistance of the heater wire at a prescribed level. Further, in order to receive radio waves by a prescribed antenna pattern, it is necessary to set the resistance of the antenna wire at a prescribed level. The resistance of the conductive printed wire depends on the line width or line thickness (layer thickness), and the thinner the line width or the line thickness is, the higher the resistance is.
On the other hand, in order to sufficiently remove defogging or to receive radio waves with a desired sensitivity over the entire region of the window, it is necessary to contrive a pattern for heater wires or antenna wires. By a computer simulation, it is possible to predict to some extent how much fogging can be removed or what grade of antenna performance can be obtained by such a pattern. Further, it has been proposed to simply adhere a conductive tape on a glass plate surface to preliminarily measure various performances (see, e.g. Patent Document 1). However, in order to obtain the final judgment as to desired heat generation performance and antenna performance, it is necessary to actually provide a conductive printed wire and measure the respective performances.
Accordingly, there may be a case that even after a screen has been prepared according to almost the final decision and a glass plate with a conductive printed wire has been produced with the screen, the pattern of the conductive printed wire has to be changed. In such a case, the screen has to be modified to meet the modified pattern.
Automobiles are mass production products, and likewise window glass plates to be used for automobiles are mass production products. Accordingly, once a pattern is determined for conductive printed wires, it is required that a conductive paste is sequentially printed on a large quantity of glass plates in the predetermined pattern. In such mass production, screen printing of a conductive paste by means of a screen is suitable. However, as mentioned above, even if a screen having a pattern substantially determined is prepared, it will be necessary to modify the screen to have the pattern adjusted to make the heat generation performance or antenna performance to be finally desired. In addition, in a case that the glass plates are to be used for windows of automobiles, the shapes of the glass plates, the shapes of patterns of conductive printed wires etc. may vary depending upon the types of automobiles. Accordingly, depending upon the types of automobiles, screens will have to be prepared, and many screens will have to be stocked. Thus, it is desired to develop a process for producing glass plates with conductive printed wires, whereby no modification of a screen is required.
In a case that a conductive printed wire is to be provided on a window of an automobile, it is required to make the line width thin as possible to secure a good view, and accordingly, it is necessary to form a conductive printed wire whose resistance is as low as possible even when the line width is smaller. For this, attempts have been made to reduce the resistance of a conductive printed wire by making the line thickness larger in the form of thick film. For example, in recent years, there have been proposed various methods to print a toner (ink) comprising conductive fine particles made of metal such as silver and a thermoplastic resin on an inorganic substance by an electro printing method, followed by firing to form a pattern of conductive wires, and to increase the thickness of a printed wire in the form of thick film by controlling conditions of printing. However, when a toner layer was formed in the form of thick film (having a line thickness of at least 20 μm) by these methods, there was a problem that difference in thermal conductivity in a thickness direction, produced at the time of firing is not negligible, so that adhesion between the conductive printed wire and the inorganic substance after firing might be insufficient. In addition, there was a danger that cracks might result in the conductive printed wires by firing, and the electric performance (resistance) of the formed pattern deteriorated largely. On the other hand, in a case that the thickness of a conductive printed wire is to be increased in the form of thick film, the particle size of conductive fine particles will generally be increased. However, the larger the particle size of the conductive fine particles is, the larger the roughness of the line width of the conductive printed wire is, and accordingly, there was a problem that it was difficult to obtain a conductive printed wire of stable resistance. Therefore, it was very difficult to produce a conductive printed wire in the form of thick film, which is usable sufficiently as a pattern of wire and which has a stable electric performance.
Patent Document 1: JP-A-2003-188622 (Claims)